In the past, Japan has suffered from a vast number of great earthquakes, for instance, one of them struck Kanto area including Tokyo in 1923, another originated in the Japan sea and still another offshore in the Tokai district. These and all other big earthquakes killed and hurt many people and destroyed buildings and railways or other various structures, giving unestimated damages in the shocked areas. In order to avoid damages by an earthquake, many attempts have been made to foresee the occurrence of an earthquake in various aspects, for example, in view of measurement of crustal deformation in different places, volcanic activities or geophysical variations including changes of geomagnetism or earthcurrent. For the same purpose, there has been established in Japan a central forecasting system for anticipating the occurrence of a possible great earthquake in or near the territory of Japan.
To realize the geophysical mechanism of seismic shocks, seismologists have been trying to investigate the theories on origins of earthquakes in which it is said that so-called "Plate Tectonics" theory is the most promising in the seismology based on the elastic repulsion in the upper region of the crust. In accordance with this theory, the geophysical mechanism of seismic occurrence is that, when a plate constituting a part of the upper crust of about 40 kilometers in thickness slowly moves and creeps under the Japan archipelago, it pulls down a marginal portion of the archipelago crust, accumulating the elastic energy which subsequently causes the crust to rebound due to the elasticity upon release from the plate over a certain limitation of the elastic energy against the frictional force between the crusts, and to thereby bring abouut a great earthquake. Accordingly, this theory obviously means that all of the earthquakes should accidentally break out with faults in the crust of about 40 kilometers in thickness.
The above-mentioned theory involves an assumption that some crust variation occurs just before a great earthquake happens, and for that reason, seems to be rather unsuitable to predict a somewhat smaller earthquake of the magnitude less than 7 which may nevertheless cause a considerable damage. On the other hand, if the crust variation is measured under this theory, the observation network would require, in practice, a vast budget for installations and costs in observation of the crust variation, since the installations should be attached on the deep sea floor or in the deep underground without clear assurance of obtaining exact observation data.
Therefore, an object of the present invention is to provide an earthquake forecasting method capable of foreseeing occurrence of a future earthquake and the epicenter in a simple manner without expensive installations and many observers.